Field of the Invention
The invention relates to a steam generator having a combustion chamber for fossil fuel, downstream of which a vertical gas flue is arranged on the heating-gas side via a horizontal gas flue.
Steam generators are normally used to evaporate a flow medium, for example a water-water/steam mixture, conducted in an evaporator circuit. To this end, the steam generator has evaporator tubes, the heating of which leads to the evaporation of the flow medium conducted therein.
Steam generators are normally constructed with a combustion chamber in an upright type of construction. This means that the combustion chamber for a throughflow of the heating medium or heating gas is designed in an approximately vertical direction. A horizontal gas flue may thereby be arranged downstream of the combustion chamber on the heating-gas side. The heating-gas flow is deflected into an approximately horizontal flow direction at the transition from the combustion chamber to the horizontal gas flue. However, the upright type of construction of the combustion chamber, on account of the temperature-induced changes in length of the combustion chamber, i.e., the heat expansion, requires a framework on which the combustion chamber is suspended. This requires considerable technical outlay during the manufacture and installation of the steam generator. This technical outlay is all the greater, the greater the overall height of the steam generator is.
The object of the invention is to provide a fossil-fuel-fired steam generator which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which requires especially little outlay in terms of manufacture and installation.
With the above and other objects in view there is provided, in accordance with the invention, a once-through steam generator, comprising:
a combustion chamber for combusting a fossil fuel;
a horizontal gas flue communicating with the combustion chamber on a heating-gas side thereof;
a vertical gas flue communicating with the combustion chamber via the horizontal gas flue; and
a plurality of burners disposed to combust the fossil fuel in the combustion chamber at a level of the horizontal gas flue.
The invention is based on the recognition that a steam generator which can be constructed with especially little outlay in terms of manufacture and installation should have a holding structure which can be constructed with simple means. In this case, a framework, which is to be constructed with comparatively little technical outlay, for the suspension of the combustion chamber can be accompanied by an especially low overall height of the steam generator. An especially low overall height of the steam generator can be achieved by the combustion chamber being given a horizontal type of construction. To this end, the burners are arranged at the level of the horizontal gas flue in the combustion-chamber wall. The heating gas therefore flows through the combustion chamber in an approximately horizontal direction during operation of the steam generator.
The burners are advantageously arranged on the end face of the combustion chamber, that is on that side wall of the combustion chamber which is opposite the outflow opening to the horizontal gas flue. A steam generator of such design can be adapted to the burn-out length of the fuel in an especially simple manner. Burn-out length of the fuel in this case refers to the flue-gas velocity in the horizontal direction at a certain average flue-gas temperature multiplied by the burn-out time tA of the fuel. In this case, the maximum burn-out length for the respective steam generator is obtained during full-load operation of the steam generator. The burn-out time tA is in turn the time which, for example, a pulverized-coal grain of average size requires in order to burn out completely at a certain average flue-gas temperature.
In order to keep material damage and undesirable contamination of the horizontal gas flue, for example on account of ash deposit, at an especially low level, the length of the combustion chamber, which is defined by the distance from the end face to the inlet region of the horizontal gas flue, is advantageously at least equal to the burn-out length of the fuel during full-load operation of the steam generator.
In accordance with an added feature of the invention, the length L (specified in m) of the combustion chamber is selected as a function of the BMCR value W (specified in kg/s) of the combustion chamber, the burn-out time tA (specified in seconds s) of the fuel and the outlet temperature TBRK (specified in xc2x0 C.) of the working medium from the combustion chamber. BMCR stands for boiler maximum continuous rating, and the BMCR value is the term normally used internationally for the maximum continuous output of a steam generator. This also corresponds to the design output, that is the output during full-load operation of the steam generator. In this case, at a given BMCR value W, the length L of the combustion chamber is approximately the larger value of the functions:
L(W, tA)=(C1+C2xc2x7W)xc2x7tA and
L(W, TBRK)=(C3xc2x7TBRK+C4)W+C5(TBRK)2+C6xc2x7TBRK+C7
where
C1=8 m/s
C2=0.0057 m/kg
C3=xe2x88x921.905xc2x710xe2x88x924 (mxc2x7s)/(kgxc2x0 C.)
C4=0.2857 (sxc2x7m)/kg
C5=3xc2x710xe2x88x924 m/(xc2x0 C.)2 
C6=xe2x88x920.8421 m/xc2x0 C.
C7=603.4125 m.
In this case, the terms xe2x80x9capproximatelyxe2x80x9d or xe2x80x9csubstantiallyxe2x80x9d are to be understood as allowing an admissible deviation by +20%/xe2x88x9210% from the value defined by the respective function.
The end face of the combustion chamber and the side walls of the combustion chamber, of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically arranged evaporator or steam-generator tubes which are welded to one another in a gastight manner and to which in each case flow medium can be admitted in a parallel manner.
For especially good heat transfer of the heat of the combustion chamber to the flow medium conducted in the evaporator tubes, a number of evaporator tubes, on their inside, in each case advantageously have ribs forming a multi-start thread. In this case, a helix angle xcex1 between a plane perpendicular to the tube axis and the flanks of the ribs arranged on the tube inside is advantageously less than 60xc2x0, preferably less than 55xc2x0. This is because, in a heated evaporator tube designed as an evaporator tube without inner ribbing, a so-called smooth tube, the wetting of the tube wall can no longer be maintained starting from a certain steam content. If there is a lack of wetting, there may be a tube wall which is dry in places. The transition to such a dry tube wall, in a type of critical stage of the heat transfer, results in an especially restricted heat-transfer behavior, so that in general the tube-wall temperatures at this location increase to an especially pronounced extent. In an inner-ribbed tube, however, this critical stage of the heat transfer, compared with a smooth tube, does not occur until there is a steam mass content  greater than 0.9, that is just before the end of the evaporation. This may be attributed to the swirl which the flow undergoes due to the spiral-shaped ribs. On account of their different centrifugal forces, the water portion is separated from the steam portion and forced onto the tube wall. As a result, the wetting of the tube wall is maintained up to high steam contents, so that there are already high flow velocities at the location of the heat-transfer critical stage. This produces especially good heat transfer and consequently low tube-wall temperatures.
In accordance with an advantageous feature of the invention, adjacent evaporator or steam-generator tubes are welded to one another in a gastight manner via metal bands, so-called fins. The fin width influences the heat input into the steam-generator tubes. The fin width is therefore preferably adapted as a function of the position of the respective evaporator or steam-generator tubes in the steam generator to a temperature profile which can be predetermined on the gas side. In this case, the predetermined temperature profile may be a typical temperature profile determined from empirical values or also a rough estimation, such as a stepped profile for example. Due to the suitably selected fin widths, heat input into all the evaporator or steam-generator tubes, even during highly inhomogeneous heating of the various evaporator or steam-generator tubes, can be achieved in such a way that temperature differences at the outlet of the evaporator or steam-generator tubes can be kept especially small. In this way, premature material fatigue is reliably prevented. As a result, the steam generator has an especially long service life.
In accordance with a further advantageous refinement of the invention, the inner tube diameter of the evaporator tubes of the combustion chamber is selected as a function of the respective position of the evaporator tubes in the combustion chamber. In this way, the evaporator tubes in the combustion chamber can be adapted to a temperature profile which can be predetermined on the gas side. With the effect which this has on the flow through the evaporator tubes, temperature differences at the outlet of the evaporator tubes of the combustion chamber are kept small in an especially reliable manner.
A common inlet-collector system is connected upstream of the evaporator tubes of the combustion chamber for the flow medium, and a common outlet-collector system is connected downstream of said evaporator tubes. A steam generator in this embodiment permits a reliable pressure balance between the evaporator tubes connected in parallel and thus permits an especially uniform flow through the same.
The evaporator tubes of the end face of the combustion chamber are advantageously connected on the flow-medium side upstream of the evaporator tubes of the side walls of the combustion chamber. As a result, especially favorable utilization of the heat of the burners is ensured.
A number of superheater heating surfaces which are arranged approximately perpendicularly to the main flow direction of the heating gas, and the tubes of which are connected in parallel for a throughflow of the flow medium, are advantageously arranged in the horizontal gas flue. These superheater heating surfaces, which are arranged in a suspended type of construction and are also designated as bulkhead heating surfaces, are mainly heated in a convective manner and are connected on the flow-medium side downstream of the evaporator tubes of the combustion chamber. As a result, especially favorable utilization of the burner heat is ensured.
In accordance with again an added feature of the invention, the vertical gas flue has a number of convection heating surfaces which are formed from tubes arranged approximately perpendicularly to the main flow direction of the heating gas. These tubes are connected in parallel for a throughflow of the flow medium. These convection heating surfaces are also mainly heated in a convective manner.
In order to also ensure especially complete utilization of the heat of the heating gas, the vertical gas flue advantageously has an economizer or high-pressure preheater.
The advantages achieved by the invention consist in particular in the fact that an especially low overall height of the steam generator can be achieved by the arrangement of the burners at the level of the horizontal gas flue. Thus the integration of the steam generator in a steam-turbine plant also permits especially short connecting pipes from the steam generator to the steam turbine. The steam generator has an especially compact type of construction due to the design of the combustion chamber for a throughflow of the heating gas in an approximately horizontal direction. In this case, the length of the combustion chamber is designed in such a way that especially good utilization of the heat of the fossil fuel is ensured.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a fossil-fuel-fired steam generator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.